[clang] [Driver] -include: deprecate probing .gch (PR #67084)

[Fangrui Song via cfe-commits]

MaskRay wrote:

I'd appreciate seeing concrete evidence rather than relying solely on intuition. I've considered the potential disruption of deprecating `.gch` files but haven't been able to substantiate it (let's say the concern is about autotools users). When using the regular expression `-o.*\.gch`, I've found very few examples, and it's likely that many of them are not affected since many builds place an `-include`d file alongside a `.gch` file.

FWIW https://github.com/Kitware/CMake/blob/92adbb5d8c2a043c6e6c8539e9817a75a813fa1b/Modules/Compiler/Clang.cmake#L108 sets `set(CMAKE_PCH_EXTENSION .pch)`

https://github.com/llvm/llvm-project/pull/67084